How is animal population density regulated? This is a fundamental question in population biology today. Microtine rodents undergo regular three to four year cycles of density and thus are an ideal group to study population regulation. No general agreement exists as to what mechanisms control density change in these species. However, dispersal has been implicated because small enclosed populations have abnormal demography. In addition, behavioral and genetic changes have been found during cycles. In the research that is in progress and that I will complete, the island endemic beach vole is being studied in detail. It occurs on the one square mile Muskeget Island, Massachusetts. Thus a natural experiment exists to determine if the population cycles and what the consequences of that answer is. Demography, reproduction, local movement, genetics, and behavior are being emphasized, and mainland controls of the meadow vole are also being studied. The outcome of this study will give us a better functional definition of a population, and add to our understanding of population regulation in general and spceifically in voles that are sometimes classified as pest species.